Philosophiae Naturalis Principia Mathematica/Motion of very small bodies when agitated by centripetal forces tending to the several parts of any very great body
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[edit] PROPOSITION XCIV. THEOREM XLVIII.
If two similar mediums be separated from each other by space terminated on both sides by parallel planes, and body in its passage through that space be attracted or impelled perpendicularly towards either of those mediums, and not agitated or hindered by any other force; and the attraction be every where the same at equal distances from either plane taken towards the same hand of the plane; I say that the sine of incidence upon either plane will be the the sine of emergence from the other plane, in a given ratio.
CASE I. Let Aa and Bb (Fig. 1) be two parallel planes, and let the body light upon the
